When exploring a prospective off-shore oil or gas field, it is customary to use a temporary, portable drilling and production rig of a type commonly known as a jack-up rig. These rigs usually have a floatable deck structure and three or four legs that can be raised and lowered relative to the deck. Thus, the rig can be floated to the drilling site where the legs are lowered to the ocean floor and the deck is raised above the water surface. Ballast tanks carried by the deck are then filled to increase the weight of the rig and thereby set the legs.
Temporary rigs most often rest on spud cans, which are large tank-like structures secured to the bottom ends of the legs. Alternatively, a "mat" may be used, this being a structure that joins the bottom ends of the legs and likewise rests on the ocean floor.
One problem associated with conventional jack-up rigs is that the spud cans tend to sink into the ocean floor, particularly if the rig is left in one position for a long period. It is then extremely difficult to raise the spud cans or mat and float the rig, even after the ballast tanks have been emptied.
Another problem that has potentially more serious consequences arises from the fact that one or more of the spud cans will sometimes break through the strata on which a rig initially rests and sink rapidly to the next high density strata. The rig can then start to lean precipitously, imposing high bending moments on one or more legs. An extremely dangerous condition results. It has also been found that the action of the water sometimes causes scouring in the area around the legs, washing away the surrounding soil and leading to further instability.
Jack-up rigs used for exploration are moved fairly frequently to drill and sample conditions in different areas, usually after one to six months at a single location. It has not been practical in the past to anchor them in the manner of permanent towers that remain in place for periods of several years or longer. These permanent towers are anchored by piles driven along or through the tower legs into the soil below. Usually the piles are driven by an air or hydraulically operated hammer held by a crane on the deck of the tower. More recently, underwater hammers have also been used for this purpose. Anchoring a permanent tower in this way is highly time-consuming and expensive, even in relatively shallow water, and the tower is not usually or readily removed and relocated.
An objective of the present invention is to overcome the above problems by adapting temporary rigs to be anchored by piles. A further objective is to use such piles in a manner that is compatible with the temporary and portable nature of the rigs, thus overcoming the previous objections to the use of piles for this purpose.